27 December 2010

Hermits and robbers

It’s a shame that fallen leaves, fruit and other natural debris are regarded as litter by those charged with cleansing local coastlines. The flotsam of high tides – loose wracks that harbour minute rafters, waterborne seeds and propagules that have drifted from afar – also receive a welcome no warmer than that given to the inorganic waste of modern life. Besides defending the shores from natural invaders, the broom brigade has effectively turned most beaches, particularly those favoured by apes who hate to share the sea with other beings, into scenes of Spartan austerity, deserts of foreign quartz that house castles of sand and sallow dreams.

There are thus few coves where indigenous clean-up crews still thrive, for their bed, breakfast and board are swept away each morn by minds who brook no mess, while those who survive the predations of terrestrial pets offer the temptation of a comical companion that costs little more than a laugh. Beyond the high water mark, there is scant hinterland for land hermit crabs to hide in, for the faint sensibilities of weekend warriors demand a landscape of party pits and paved paths before a park is deemed fit for human nature.

Only on offshore isles do coenobitids still prevail to amble with impunity as they once did on mainland strands. They are possibly the dominant land fauna on islets by maritime fairways, and still occur in good numbers on slightly larger refuges that few in this age of cheap flights resort to for a measure of paradise. Come twilight, the crabs emerge from invisible shelters to prowl the lawns for leftovers and lesser treats. Some pack their soft abdomens in the robust shells of deceased turbans, moons and melongenas, while others appear content to sport the lighter whorls of alien snails. They appear ungainly, but have enough strength and sureness of feet to ascend the lower reaches of coconut trees and scale the tall buttresses of sea almonds.

When threatened, most land hermit crabs retreat into their portable fortresses, using their enlarged and reinforced left chelae to plug the aperture and protect their vulnerable parts. One coenibitid, however, has outgrown the need to flee by attaining the size of a monster in miniature. Called the robber crab for its reputed penchant for raiding palm groves, Birgus latro is a lumbering beast that infests the cliff-lined beaches and moist terrace forests of oceanic islands across the Indo-Pacific, though it is strangely absent from much of the Sunda shelf. Hunting has reduced their numbers in most traditional habitats, save in refugia such as Christmas Island, which harbours the world’s largest population.

Youngsters who survive their planktonic stage hoist shells that match their mass, but at some point, the crabs shed all pretence of fragility and wander in brazen nakedness, using their claws as ambulatory appendages as well as accessories for tearing open carcasses and coconuts. Curiously, the robber crabs we encountered by the trails prefer to ward off our interest by lashing out with their walking legs, welding their fearsome chelae not as arms but as armour for their faces. This puerile defence is also why drivers there are warned to skirt rather than straddle jaywalking robbers to avoid the risk of damage to both crab and car. Despite their bulk, the giant anomurans are excellent climbers; we found many perched on the limestone pinnacles that scar the island’s rainforests, though only one was spotted clinging to a sapling near the Dales. The crabs gather in unnerving numbers at certain patches of forest, lured perhaps by the pith of a favoured endemic palm. Unguarded picnic baskets, according to one source, also attract gangs that emerge from the undergrowth like a plague of zombie roaches.

At least three other coenobitids, including a tawny species common to Singapore, share the island, but we encountered none of these amid the distracting tide of red that forages on nearly every square metre of forest. With few mammals and reptiles to rule the roost, Christmas Island’s land crustaceans have formed a pecking order of their own, with nippers and blue crabs providing ineffectual population control of their scarlet kin and Birgus riding roughshed over all. Having escaped the infinite hazards of life in the littoral zone, hermits and their fellow wanderers have claimed a stake on tropical woods only to fall victim to an appetite for land that has little room to spare for the daily rituals of this awkward tribe.